Electromechanical transducer system



' Aug. 20, 1963 s. R. RICH 3,101,419

ELECTROMECHANICAL TRANSDUCER SYSTEM Original Filed Aug. 27, 1956 2. Sheets-Sheet 1 RELATIVELY HIGH'DENSITY MATERIAL I 22 ,2

. ,flO 23 ng A 23 -2| 25- 26- 3 H62 1 I I I I l4 As 1 |7 A n -2 a 4 6 3 I8 fi 7 4o ,3 RELATIVELY/ HG LOW DENSITY |G| MATERIAL F as as F IG.4

INVENTOR. STANLEY R. RICH ATTORNEY I 1933 s. R. RICH 3,101,419

ELECTROMECHANICAL TRANSDUCER SYSTEM Original Filed Aug. 27, 1956 2 She t -Sh et 2 STRESS OR STRAIN c 2Q FIG.6 I A MPLITUDE VELOCITY OR ACCELERATION W! //lll/ INVENTOR.

STANLEY R. RICH ATTORNEY Stanley R.

U it I States Patent 3,101,419 ELECTROMECHANICAL TRANSDUCER SYSTEM Rich, Hartford, Conn, assignor, by mesne assignments, to Acoustica Associates, Inc, Los Angeles,

Calif., a corporation of New York Original application Aug. 27, 1956, Ser. No. 606,478, new

Patent No. 2,947,889, dated Aug. 2., 1960. Divided and this application Apr. 21, 1960, Ser. No. 23,805

19 Claims. (Cl. 3108.3)

This invention relates to electromechanical transducers, and .more particularly to systems employing such transducers to interchange large quantities of energy with low losses. This application is a division of Serial No. 606,- 478, filed August 27, 1956, now Patent No. 2,947,889, issued August 2, 1960.

Uses of elastic wave energy in industry are being made and innovated at an increasing 'pace. However, in some areas, such as processes employing liquid baths which it is desired to irradiate with compressional Wave (sonic or ultrasonic) energy, progress into large scale use of such energy awaits the availability of an electromechanical transducer at reasonable cost which will convert large amounts of electrical energy into compressional wave energy, with small and insignificant heat loss. Known transducers generally lack the ability to handle large power, except perhaps as pulse peaks, without soon generating destructive heat. Efliciencies below 50 percent are common, and efficiencies as much as 65 percent are considered high. Further, the permissible stresses and strains that can be endured by the electromechanical transducer material are so low that these materials are soon destroyed by the amounts of power which can be usefully employed in such processes. Attempts to solve this problem by employing more transducers often carries the cost of an installation beyond economically acceptable limits.

It is an object of the present invention to provide an electromechanical transducer system which is capable of interchanging large amounts of electrical and elastic wave energy with high efliciency. It is another object of the invention to provide such a system having a relatively low Q so that it can be used with electric energy sources which donot have reliable frequency regulation, such as a motor generator driven by an induction motor. It is a further object of the invention to provide such a system which can operate continuously without generatingdestructive heat in any of its parts. These and other desirable objects are attained, according to this invention, by a sandwich transducer structure in which a thin electromechanical transducer is held between two pieces of solid materials having relatively dissimilar densities, for

example aluminum and brass, or aluminum and steel. According to another important feature of the invention, this structure is held together by suitable clamping means with a force sufiicient to apply a static compressive stress of a magnitude greater than and in opposition to any instantaneous negative stress produced in the system during vibration at practically any energy level. These features of the invention are described in greater detail in the following description of certain embodiments. This description refers to the accompanying drawings, in which: i

FIG. 1 is a vertical section through a transducer system built according to the invention;

FIG. 2 is :a top view of FIG. 1;

FIG. 3 is a section along line 3-6 of FIG. 1;

FIG. 4 is a fragmentary vertical section showing a modification of FIG. 1;

FIG. 5 illustrates in detail an electromechanical transducer element suitable for use in systems built according to the invention;

3,1 M ,4 l 9 Patented Aug. 20, 1963 r ce FIG. 6 is a graph to aid the description;

FIG. 7 is a vertical section, partly broken away through another embodiment of the invention; and

FIG. 8 illustrates another electromechanical transducer element suitable for use in systems built according to the invention.

In FIG. 1 a block of steel or other suitable metal of corresponding properties It) and a block of aluminum or other suitable light metal 11 have four electromechanical transducer elements, 12, 13, 14 and 15 as shown in FIG. 3, sandwiched between them. These maybe made of any piezoelectric or electrostrictive material, for eX- ample quartz, or barium titanate. This sandwich is held together by a bolt 17 passing through bores in the blocks 10 and 11. The head 18 of the bolt 17 rests on 'a shoulder provided by enlarging the .outer portion 19 of the bore in the aluminum block 11. The outer portion 21 of the bore in the steel block 10 is enlarged to receive an electrically insulating sleeve 22 and the nut 23 of the bolt 17. A shoulder 25 is provided at the junction of the outer portion 21 and the inner portion 26 of the bore in the steel block 10, and an electrically insulating washer 27 rests on this shoulder. A second electrically insulating sleeve 28 lines the inner bore portion 26. A convexsh-aped hard metal washer 30 rests on the washer 27, and the nut 23 is used to compress this washer to apply a static compressive stress to the system. The magnitude of this stress, and its purpose are described below.

, Referring to FIG. 4, the elements corresponding to elements shown in FIG. I bear the same reference numbers.

' The transducer elements (only elements 14. and 15' are p of equal 330 cycles per second in most liquids.

shown) in this embodiment are aflixed to the blocks 10 and 11 by a suitable cement 35 An epoxy cement is suitable. Each transducer element is fitted with electric conductors, one for each face. The conductors 36 and 37 for element 15 are shown only, and these are connected one to each of blocks 10 and 11. As indicated in FIG. 5, each transducer element is provided with an electrode 38 on each face; each electrode has a tab to which one of the wires 36 or 37 is attached. While in FIG. 1, the blocks 10 and 11 themselves make electrical contact with the transducer elements, which are usually fitted with electrodes like electrode 38, these elements can if desired be fitted with conductors like conductors 36 and 37, and the conductors canbe bonded to the blocks as shown in FIG. 4, to insure direct electrical contact to the electrodes in all events. On the other hand, the conductors 36 and 37 may be omitted entirely from the embodiment shown in FIG. 4. I have built such systems, in which the static compressive stress was applied whilethe cement 35 was soft, and they have operated successfully.

The sandwich technique. of transducer system con- 3 of British patent specification 145,691, for example. In the Langevin syst two steel or stainless steel blocks thickness are placed one on each side of a mosaic of quartz crystals. The system resonates as a half-wave vibrator with the quartz (which is comparatively thin) located essentially at a node. The over-all Q of this device is cxteremely high, being basically the ratio between the acoustic impedance of steel and the acoustic is of the order of 30. That is, the frequency span of op ra- 'tion is & of the resonant frequency. At 20 kc./sec., the Langevi-n transducer must be operated at plus or minus This has too high a Q for industrial purposes.

In the present invention, a low density (light) metal is used for one side of the sandwich, and a high density (heavy) metal is used for member side. Energy is radiated from the surface of the light metal. One efiect of this new structure is a reduction in Q, due to the fact that the acoustic impedances ot light metals (aluminum or magnesium) are of the order of to V that of steel, to values of Q of the order of 10. Conservation of momentum considerations still further reduces the Q of the over-all system by virtue of the fact that the surface of the light metal must move with a particle displacement of many times the particle displacement at the surface of the heavy metal. The ratio is the ratio of the densities of the two metals, if the velocity of sound is the same in both. The reason for this is that the velocity of sound determines the resonant frequency of the sandwich while the particle velocity is determined by the fact that the momentum of the two sides must be the same, to satisfy the requirements of the law of conservation of momentum. This dictates that the particle velocities at the end surfaces of the two blocks and 11, for example, must be in inverse ratio to the densities of the two materials of the blocks, in order that the density of one metal times the particle velocity at its surface may be equal to the density of the other metal times the particle-velocity at its surface. In sandwich combinations of steel or brass for example as the heavy metal with aluminum or one of its alloys as the light metal, a particle displacement or velocity amplification of about 3.33 is obtained, and when magnesium or one of its alloys is substituted tor aluminum or its alloys the particle velocity or displacement amplification is about 5.6. The Q is thus further lowered because energy radiated tromjthe surface of the lower density metal is greater per unit energy stored in the resonant structure.

Referring to FIG. 6, which illustrates schematically a halt-wave longitudinal vibrator 42, curve A indicates the amplitude, velocity or acceleration conditions at various particle positions in the vibrator if it is constructed according to Langevin, namely, a sandwich with both :free ends made ofsolid materials having substantially the same density. These conditions are maximum and equal and opposite at the ends, passing through zero at the node, in the center. Curve B illustrates the particle conditions provided by the present invention, that is, when the system is constructed as a sandwich in which the outer solid materials have relatively different densities, or acoustic impedances. Assuming the left-hand end to be that of the lower density metal, the maximum amplitude, velocity or acceleration of a particle at the left-hand end of the vibrator 42 is greater than at the right-handend, and greater for the same input energy than in the prior art structures.

Momentum considerations in the present case cause the lighter metal, which'is used as the radiating surface,

' to have greater particle velocity than the heavy metal,

as shown in FIG. 6. This means that greater energy is available for radiation because radiated energy is proportional to the square of particle velocity. Consequently the present invention, as distinguished from the prior :art devices which distribute available energy equally to both halves of the sandwich, provides a far greater proportion of the total energy to the radiating (and lighter) part than to the nonradiating (heavier) side.

The radiated energy can be made to leave the transducer system (at the bottom surface 40 in FIG. 1, for example) in a plane-wave term. This is often desirable because it can avoid undesirable [focusing effects in treatment baths, and thereby permit accurately controlled irradiation of such baths with arrays of transducer systems. As an example of dimensions suitable for this purpose, one embodiment of a half-wave 20-kc./sec. vibrator uses steel and aluminum alloy cubes 2" X 2. x 2", with barium titanate A" thick between them. These dimensions of the cubes all approximate to one-quarter wave length of elastic waves in the material. If brass or other copper-bearing metal is used in place oi the steel, its dimensions may be 2 x 2 x 1 /2", since brass and other copper-bearing metals have a lower velocity of sound than aluminum and 1 /2" depth is equivalent approximately to 2" in aluminum. The aluminum alloys used are alloys containing at least 70 percent aluminum.

While sandwich structures held together solely by cement will work and give all the amplification effects described above, consideration of curve C in FIG. 6 will show that the stress or strain in a half-wave vibrator is maximum at the node. If the cemented surfaces are so located in the structure, whether it be one-half wave long or longer, the strength of the cement bond limits the power that can be developed by the system.

When power is increased, eventually an instantaneous negative stress will be reached which will be capable of weakening or rupturing the bond. The bolt 17 and nut 23 are preferably tightened on the washer 30 to exert a static positive compressive stress which is greater in magnitude than any instantaneous negative stress possible at desired power levels of operation. This makes it possible greatly to extend both the power range and the useful life of sandwich transducer systems. The bonding material is preventedfrom going through the type of 'mechanical hysteresis loop that occurs in nonengineering materials, which results in internal losses and eventual failure. As shown in FIG. 1, the bonding material may be omitted entirely, when compressive stress is provided, if all meeting surfaces are accurately flat. I have built such transducer systems, and operated them successfully.

The efficiency of transducer systems according to the invention is very high, exceeding 90 percent, when barium titanate is used, "for example. Ordinarily barium titanates, and certain ferrites, have efficiencies around 65 percent when operated in a conventional thickness mode. The losses are mainly mechanical (in barium titanate the electrical losses are less than 4 percent, as a capacitor). However, in the present invention, the transducer material (e.g., barium titanate) is only a small fraction of the vibrating system. For example, in a 20-kc./sec. systern, it may be only A out of 4", or 6.3 percent of.

the structure. Relatively lossless metals or other materials then make up 94 percent of the total. The generated heat, or losses, are quite small. Also, whatever heat is generated in the transducer elements is easily given up to the metal masses because the elements are thin, and is easily carried away by the metal masses.

An alternative transducer system is shown in FIG. 7, in which a clamping means for applying static compressive stress to the system is located outside the system. Blocks '50 and 51 corresponding respectively to blocks 10 and 11 in FIG. 1, hold between them transducer elements 54 and 55 corresponding respectively to elements 14 and 15 in FIG. 1. Flanges 60 and 61 are provided on the left-hand side of each block, 50 and 51 respecthe opposite side of each block. Each set of confronting flanges is provided with bores, 62 and 63, and the bore 62 in flange 60 is fitted with an electrically insulating sleeve 65. A bolt 67 is fitted in the bores and through I the sleeve 65, and a curved metal washer 68, resting on an electrically insulating washer 69, is compressedby a nut 71 to apply the static compressive stress. 'It will be appreciated that at least two sets of flanges and bolts are needed to apply this force, although to distribute it as evenly as possible more may be desired in a particular case. Obviously, cement can be included according to FIG. 4, if desired.

FIG. 8 illustrates the structure of a magnetostrictive transducer element which can be substituted in FIG. 1, FIG. 4 or FIG. 7, for the elements shown in FIG. 3. A

block of magnetostrictive material is provided with thickness direction, for passage of the bolt 17 if a structure like that'of FIG. 1 is used.v The block 80 canbe made of laminated sheets, or can be solid, as shown.

' It can be a ferrite. When magnetostrictive transducer electromechanical transducer means sandwiched between two pieces "ofsolid materials which have relatively dissimilar densities and constituting with said pieces a me? chanical vibrator, and means to apply to said system a static -compressive stress in opposition to any instan-- taneous negative stress produced in said system during 1 vibration at an energy level sufficiently great to produce an instantaneous negative stress large enough to disrupt the acoustic bond one of said pieces in the absence of said static compressive stress, said vibratorbeing dimensioned to vibrate substantially as a longitudinally resonant vibrator in' the direction of a line through said pieces and said trans ducer means.

2. An electromechanical transducer system of the expander typein which the dimension parallel to which vibration occurs during operation is such that the system is adapted to vibrate in resonance at a given frequency comprising electromechanical transducer means sandwiched between two pieces of solid materials which have relatively dissimilar densities arrayed'along said dimension, thedimension of said electromechanical transducer means parallel to said first-named dimension being a minor fraction of'a half-wave of vibration @of said system, said electromechanical transducer means being located substantially at a vibration node of said system, and means to apply to said system a static compressive stress in opposition to andof a magnitude greater than any instantaneous negative stress which would be produced in said system at an interface between said electromechanical transducer means and said pieces during the applicationof electrical driving signals at any desired energy level to said electromechanical transducer means.

3. An electromechanical transducer system comprising electromechanical transducer means sandwiched between two, pieces of solid material and constituting with said pieces a mechanical vibrator adapted to vibrate in the directionofa line through said pieces and said transducer means, a cement bonding said transducer means tothe confronting face of each of said solid pieces, and means to apply to said system a static compressive stress parallel to said line in oppositionto and ofa magnitude at least equal to any instantaneous negative stress which would be produced in said system at an interface between said transducer means andsaid pieces during vi bra-tion at any desired energy level sufiiciently great to rupture the bond of said cement, said transducer means having at least one interface with said pieces located substantially at the position of a vibrational node in said system. 1 4. An electromechanical transducer system comprising electromechanical transducer means sandwiched between two pieces of' solid materials having relatively dissimilar densities and constituting with said pieces a mechanical vibrator adapted to vibrate in the direction of a line through said pieces and said transducer means, a cement bonding saidtransducer means to the confronting face of each of said solid pieces, and meansto apply to said system a static compressive stress parallel to said line in opposition to and of a magnitude at least equal to any instantaneous negative stress which would be pro duced in said system at an interface between said transbetween said transducer means and ducer means and said pieces during vibration at any energy level sufiiciently great to rupture the bond of said cement, said transducer means having at least one intertface with said pieces located substantially at the position of a vibrational node in said system.

5. An electromechanical transducer system comprising electromechanical transducer material interposed between two pieces of solid material and constituting with said pieces a mechanical vibrator dimensioned to vibrate at the intended operating frequency substantially as a longitudinally resonant half-wave vibrator in the direction of a line through said pieces and said transducer material such that during vibration of said system in the longitudinal mode at said operating frequency antinodes of longitudinal elongation and contraction occur substantially'at each endof the system and a node occurs between said ends, each of said pieces having a bore passing through it in the direction of said line, said bores being collinear, each bore having a portion at the outer end of said system which is of larger diameter than the remainder thereof and having a shoulder between the larger and smaller diameter portions thereof, a passage through said transducer material in register with said bores, a bolt having at one end a head having a transverse dimension larger than either smaller bore diameter and being threaded at the other end, said bolt passing through said bores and passage and traversing said node, with said head in one of said outer bore portions and disposed to bear on the adjoining shoulder, and a threaded portion of said other end in the other of said outer bore portions, a threaded not on said threaded portion of said bolt and disposed to bear on the other of said shoulders, and annular rcsilientmeans having a convex shape in the relaxed condition surrounding said bolt and disposed to be compressed toward a flattened shape eifectively against one of said shoulders when said nut is tightened, to apply to said system through said shoulders a compressive stress having a prescribed static magnitude, the length of said bolt between said shoulders being substantially less than the distance between said ends of said system whereby said bolt will not self-resonate in a longitudinal mode system.

6. An electromechanical transducer system comprising electromechanical transducer material interposed between two pieces of solidmaterial and constitutingwith said pieces a mechanical vibrator dimensioned to vibrate at the intended operating 7 I a line through said pieces and said transducer material such that during vibration of said system in the longitudinal mode at sa d operating frequency antinodes of longiends, each of said pieces having a bore passing through it in the direction of said line, said bores being collinear,

ducer material in register with said bores, a bolt having at one end a head having a transverse dimension larger than either smaller bore diameter and being threaded at the other end, said bolt passing through i said shoulders a compressive stress having a prescribed" static magnitude, the length of said bolt between said' at the operating frequency of said frequency substantially as a longi tudinally resonant half-wave vibrator in the direction of I said outer bore portions, a threaded nut on said threaded portion of said bolt and disposed shoulders being substantially less than the distance between said ends of said system whereby said bolt will not self-resonate in a longitudinal mode at the operating frequency of said system.

7. An electromechanical transducer system comprising electromechanical transducer material interposed between two pieces of solid material and constituting with said pieces a mechanical vibrator dimensioned to vibrate at the intended operating frequency substantially as a longitudinally resonant vibrator n half-waves long, where n is an integer, in the direction of a line through said pieces and said transducer material such that during vibration of said system in the longitudinal mode at said operating frequency antinodes of longitudinal elongation and contraction occur substantially at each end of the system and at least one node occurs between said ends, each of said pieces having a bore passing through it in the direction of said line, said bores being collinear, each bore having a portion at the outer end of said system which is of larger diameter than the remainder thereof and having a shoulder between the larger and smaller diameter portions thereof, a passage through said transducer material in register with said bores, a bolt having at one end a head having a transverse dimension larger than either smaller bore diameter and being threaded at the other end, said bolt passing through said bores and passage and traversing said node, with said head in one of said outer bore portions and disposed to bear, on the adjoining shoulder, and a threaded portion of said other end in the other of said outer bore portions, a threaded nut on said threaded portion of said bolt and disposed to bear on the other of said shoulders, and annular resilient means having a convex shape in the relaxed condition surrounding said bolt and disposed to be compressed toward a flattened shape effectively against one of said shoulders when said nut is tightened, to apply to said system through said shoulders a compressive stress having a prescribed static magnitude, the length of said bolt between'said shoulders being substantially less than one half-wave of vibration of said system whereby said bolt will not self-resonate in a longitudinal mode at the operating frequency of said system.

8. An electromechanical transducer system comprising electromechanical transducer material interposed between two pieces of solid material and constituting with said pieces a mechanical vibrator dimensioned to vibrate at the intended operating frequency substantially as a 1ongitudinally resonant vibrator n half-waves long, where n is an integer, in the direction of a line through said pieces and said transducer material such that during vibration of said system in the longitudinal mode at said operating frequency antinodes of longitudinal elongation and contraction occur substantially at eachend, of the system and at least one node occurs between said ends, each of said pieces having a bore passing through it in the direction of said line, said bores being collinear, each bore havingla portion at the outer end of said system which is Olf larger diameter than the remainder thereof and having a shoulder between the larger and smaller diameter portions thereof, a passage through said transducer material in register with said bores, a bolt having at one end a head having a transverse dimension larger than either smaller bore diameter and being threaded at the other end, said bolt passing through said bores and passage and traversing said node, with said head in one of said outer bore portions and disposed to bear on the adjoining shoulder, and threaded portion of said other end in the other of said outer bore portions, a threaded nut on said threaded portion of said bolt and disposed to bear on the other of said, shoulders, and resilient means cooperating with said bolt and nut and disposed to be compressed effectively against one of said shoulders.

when said nut is tightened, to apply to said system through said shoulders a compressive stress having a prescribed static magnitude, the length of said bolt between said 8 shoulders being substantially less than one-half wave of vibration of said system whereby said bolt will not selfresonate in a longitudinal mode at the operating frequency of said system.

9. An electromechanical transducer system comprising electromechanical transducer material interposed between two pieces of solid material and constituting with said pieces a mechanical vibrator dimensioned to vibrate at the intended operating frequency substantially as a longitudinally resonant n half-waves long, where n is an integer, in the direction of a line through said pieces and said transducer material such that during vibration of said system in the longitudinal mode at said operating frequency antinodes of longitudinal elongation and contraction occur substantially at each end of the system and at least one node occurs between said ends, each of said pieces having a bore passing through it in the direction of said line, said bores being collinear, each bore having a portion at the outer end of said system which is of larger diameter than the remainder thereof and having-a shoulder between the larger and smaller diameter portions thereof, a passage through said transducer material in register with said bores, a bolt having at one end a head having a transverse dimension larger than either smaller bore diameter and being threaded at the other end, said bolt passing through said bores and passage and traversing said node, with said head in one of said outer bore portions and disposed to bear on theadjoining shoulder, and a threaded ,portion of said other end in the other of said outer bore portions, a threaded nut on said threaded portion of said bolt and disposed to bear on the other of said shoulders, and annular resilient means having a convex shape in the relaxed condition surrounding said bolt and disposed to be c0n1- pressed toward a flattened shape effectively against one of said shoulders when said nut is tightened, to apply to said system through said shoulders a compressive stress having a prescribed static magnitude, the length of said bolt between said shoulders being substantially different from an integral number of half-waves of vibration of said system whereby said bolt will not self-resonate in a longitudinal mode at the operating frequency of said system.

10. An electromechanical transducer system comprising electromechanical transducer material interposed between two piebes of solid material and constituting with said pieces a mechanical vibrator dimensioned to vibrate at the intended operating frequency substantially as a longitudinally resonant vibrator n half-waves long, where n is an integer, in the direction of a line through said pieces and said transducer material such that during vibration of said system in the longitudinal'nrode at said operating frequency antinodes of longitudinal elongation and contraction occur substantially at each end of the system land at least one node occurs between said ends, each of said pieces having abore passing through it in the direction of said line, said bores being collinear,-

eaich bore having a portion at the outer end of said system which is of larger diameter than the remainder thereof and having a shoulder between the larger and smaller diameter portions thereof, a passage through said transducer material in registerwith said bores, a bolt having at one end a head having a transverse dimension larger nut on said threaded portion of said bolt and disposed to bear on theother of said shoulders, and resilient means cooperating with said'bolt and nut and disposed to be compressed effectively against'one of said shoul ders when said nut is tightened, to apply to said system through said shoulders a compressive stress having a prescribed static magnitude, the length of said bolt between said shoulders being substantially different from an integnal number of halfwaves of said system whereby said bolt will not self-resonate in a longitudinal mode at the operating frequency of said system.

11. An electromechanical transducer system compris ing electromechanical transducer material interposed be- I tween two pieces of solid material and constituting with said ends, and means to apply to said system a compressive stress having a prescribed static magnitude, said means comprising two interfitting threaded parts, at least one of said parts being elongated in the direction of said line and tnaversing said node, said parts being arranged to compress said pieces on said transducer material, the total length of said two intenfitting parts being substantially less than the distance between said ends whereby said stress applying means will not self-resonate at the operating frequency of said system, and annular resilient means having a convex shape in the relaxed condition surrounding said elongated part and disposed between respective bearing surfaces on one of said parts and one of saidpieces, said annular means, when compressed toward a flattened shape, constituting'resilient means to establish said prescribed, magnitude. I

:12. An electromechanical transducer'system comprising electromechanical transducer material interposed between two pieces of solid material and constituting with said pieces a mechanical vibrator dimensioned to vibrate at the intended operating frequency substantially as a longitudinally resonant vibrator n half-waves long, where n is an integer, in the direction of a line through said pieces and said transducer material such that during vibration of said system in the longitudinal mode of said operating frequency antinodes of longitudinal elongation and contraction occur substantially at each end of the system and at least one node occurs between said ends, and means to apply-to said system a compressive stress having )8, prescribed static magnitude, said means comprising two interfitting threaded parts, atleast one of said parts being elongated in the direction of said line and traversing said node, said parts being arranged to compress said pieces on said transducer material; the total length of said two interfitting parts being substantially diiferent from an integral number of half-waves of vibration of said systems whereby saidstress applying means will not self-resonate at the operatingfrequency of saidsystem, and annular resilient means having a convex shape in the relaxed condition surrounding'said elongated part and disposed between respective bearing surfaces on one of said parts and one of said pieces, said annular means when compressed toward a flattened shape'constituting resilient means to establish said prescribed magnitude.

13. An electromechanical transducer system for irradi- 'ating a liquid bath with elastic wave power, comprising electromechanical transducer. material interposed between confronting faces of two pieces of solid material and constituting with said pieces a mechanical vibrator dimensioned to vibrate at the intended operating frequency su l6 duce compressional wave energy into the liquid at a prescribed work-performing power level, and clamping means passing in the direction of said line from one of said pieces to the other, to apply to said system a compressive stress having a prescribed static magnitude in opposition toand greater than, any instantaneous negative stress produced in said system in the vicinity of said node during vibration of said system at an energy level sufiicient to introduce said compressional wave energy at said power level into a liquid bath acoustically coupled to said one end, said clamping means comprising two interfitting threaded parts, at least one of said parts being elongated in the direction of said line and traversing said node, and annular resilient means having a convex shape in the relaxed condition surrounding said elongated part and dis.

posed between respective bearing surfaces on one of said parts and one of said pieces, said annular means when compressed toward'a flattened shape constitute resilient means to establish said prescribed magnitude.

14. An electromechanical transducer system for irradiating a liquid bath with elastic wave power, comprising electromechanical transducer material interposed between confronting faces of first and second pieces of solid material and constituting with said pieces a mechanical vibrator dimensioned to vibrate at the intended operating frequency substantially as a longitudinally resonant vibrator n half-waves long, where n is an integer, in the direction of a line through said pieces and said transducer material such that during vibration of said system in the longitudinal mode at said operating frequency antinodes of longitudinal elongation and contraction occur substantially at each end of the system and a node occurs substantially between said confronting faces, the one of said ends at said first piece of solid material being intended. for acoustic coupling with a liquid bath to introduce compressional wave energy into the liquid at a prescribed work-performing power level, and clamping means passing in the direction of said line from one of said pieces to the other, to apply to said system a compressive stress having a prescribed static magnitude in opposition to and greater than any instantaneous negative stress produced in said system in the vicinity of said node during vibration of said system at an energy level sufficient to introduce said compressional wave'energy at said power level into in g electromechanical vibrator dimensioned a liquid bath acoustically coupled to said one end, the density of the solid material of said second piece being at least twice the density of the solid material of said first piece.

15. An electromechanical transducer system for irradiating a liquid bath with elastic wave tially ateach end of the system and a node occurs substantially as alongitudinally resonant vibrator n halfwaves long, where n is an integer, in the direction of a line through said pieces and said transducer material such that during vibration of said system in the longitudinal mode at said operating frequency antinodes of longitudinal elongation and contraction occur substantially'at stantially between said confronting faces, the one of said ends at said first piece of solid material being intended for acoustic coupling with a liquid bath to introduce compressional wave energy into the liquid at a prescribed workperforming power level, and clamping means to apply to said system a compressive stress having magnitude in opposition to and greater than any instantaneous negative stress produced in said system in the each end of the system and a node occurs vsubstantially between said confronting faces, one of said ends being intended for acoustic coupling with aliquid bath to introvicinity of said node during vibration of said system at an energy level sufficient to introduce said compressional wave energy at said power level into a liquid bath acoustically coupled to said one end, the density of the solid material of said second piece being at least twice the density of the solid material of said first piece.

is an integer, in the a prescribed static transducer material such that during vibration of said systern in the longitudinal mode at said operating frequency antinodes of longitudinal elongation and contraction occur substantially at each end of the system and a node occurs between said ends, one of said ends being intended for acoustic coupling with a liquid bath to introduce comvpressional wave energy into the liquid at a prescribed work-performing power level, and clamping means passing in the direction of said line from one of said pieces to the'other, to apply to said system a compressive stress having a prescribed static magnitude in opposition to and greater than any instantaneous negative stress produced in said system in the vicinity of said node during vibration of said system at an energy level suflicient to introduce said compressional wave energy at said power level into a liquid bath acoustically coupled to said one end, said clamping means comprising two interfitting threaded parts, at least one of said parts being elongated in the direction of said line and traversing said node, and resilient means cooperating with said elongated part and disposed between respective bearing surfaces on one of said parts and one of said pieces, said resilient means when compressed constituting resilient means to establish said prescribed magnitude, said elongated part being substantially shorter than the. distance between said ends whereby said elongated part will not self-resonate in a longitudinal mode at the operating frequency of said system.

17. An electromechanical transducer system for irradiating a liquid bath with elastic wave power, comprising electromechanical transducer material interposed between confronting faces of first and second pieces of solid material and constituting with said pieces a mechanical vibrator dimensioned to vibrate at the intended operating frequency substantially as a longitudinally resonant halfwave vibrator in the direction of a line through said pieces and said transducer material such that during vibration of said system in the longitudinal mode at said operating frequency antinodes of longitudinal elongationand contraction occur substantially at'each end of the system and a node occurs between said ends, the one of said ends at said first piece of solid material being intended for acoustic coupling with a liquid bath to introduce compressional wave energy into the liquid at a prescribed workperforming power level, and clamping means passing in the direction of said line from one of said pieces to the other, to apply to said system a compressive stress having a prescribed static magnitude in opposition to and greater than any instantaneous negative stress produced in said 7 system in the vicinity of said node during vibration of said system at an energy level sufiicient to introduce said compressional wave energy at said power level into a liquid bath acoustically coupled to said one end, said clamping means comprising two interfitting threaded parts, at least one ot-said parts being elongated in the direction of said line and traversing said node, and resilient means cooperating with said elongated part and disposed between respective bearing surfaces on one of said parts and one of said pieces, said resilient means when compressed constituting resilient means to establish said prescribed magnitude, said elongated part being substantially shorter than the distance between said ends whereby said elongated part will not self-resonate in a longitudinal mode at the operating firequency of said system, the density of the solid material of said second piece being at least twice the density of the solid material of said first piece.

18. An electromechanical transducer system for irradiating a liquid bath with elastic wave power, comprising electromechanical tranducer material interposed between confronting faces of first and second pieces of solid material and constituting with said pieces a mechanical vibrator dimensioned to vibrate at the intended operating frequency susbstantially as a longitudinally resonant halfwave vibrator in the direction of a line through said pieces and said transducer material such that during vibration of said system in the longitudinal mode at said operating frequency antinodes of longitudinal elongation and contraction occur substantially at each end of the system and a node occurs between said ends, the one of said ends at said first piece of solid material being intended 'for acoustic coupling with a liquid bath to in troduce compressional wave energy into the liquid at a prescribed work-performing power level, and clamping means passing in the direction of said line from one of said pieces to the other, to apply to said system a compressive stress having a prescribed static magnitude in opposition to and greater than any instantaneous negative stress produced in said system in the vicinity of said node during vibration of said system at an energy level suflicient to introduce said compressional wave energy at said power level into a liquid bath acoustically coupled to said one end, saidclamping means comprising two interfitting threaded parts, at least one of said parts being elongated in the direction of said line and traversing said node, and annular resilient means having a convex shape in the relaxed condition surrounding said elongated part and disposed between respective bearing surfaces on one of said parts and one of said pieces, said annular means when-compressed toward a flattened shape constituting resilient means to establish said prescribed magnitude, said elongated part being substantially shorter than the distance between said ends whereby said elongated part will not self-resonate in a longitudinal mode at the operating frequency of said system, the density of the solid material. of said second piece being at least twice the density of the solid material of said first piece;

19. An electromechanical transducer system comprising electromechanical transducer means sandwiched between two pieces of solid material and constituting with-said pieces-a mechanical vibrator adapted to vibrate in the direction of a line through said pieces and said transducer means, flanges on each of said pieces, said flanges having collinear pairs of bores, and a bolt in each of said pairs of bores to apply to said system a static compressive stress parallel to said line in opposition to and of a magnitude at least equal to any instantaneous negative stress which would be produced in said system at an interface between said transducer means and said pieces during vibration at any desired level, means to tighten said bolts to urge said flanges together, a convex resilient annular member being provided to cooperate with each bolt so that tightening of said bolt compresses said member to establish and maintain a desired magnitude of static compressive stress, said transducer means having at least one interface with said pieces located substantialy at the position of a vibrational node in said system. v v p References Cited in the file of this patent UNITED STATES PATENTS 1,907,425 Marrison May 9, 1933 2,044,000 Heising June 16, 1936 2,137,852 Nicolson Nov. 22, 1938 2,454,264 Stigter Nov. 16, 1948 2,704,333 'Calosi et al. Mar. 15, 1955 2,937,292 Welkowitz et al. May 17, 1960 McGunigle Aug. 2, 1960 

1. AN ELECTROMECHANICAL TRANSDUCER SYSTEM COMPRISING ELECTROMECHANICAL TRANSDUCER MEANS SANDWICHED BETWEEN TWO PIECES OF SOLID MATERIALS WHICH HAVE RELATIVELY DISSIMILAR DENSITIES AND CONSTITUTING WITH SAID PIECES A MECHANICAL VIBRATOR, AND MEANS TO APPLY TO SAID SYSTEM A STATIC COMPRESSIVE STRESS IN OPPOSITION TO ANY INSTANTANEOUS NEGATIVE STRESS PRODUCED IN SAID SYSTEM DURING VIBRATION AT AN ENERGY LEVEL SUFFICIENTLY GREAT TO PRODUCE AN INSTANTANEOUS NEGATIVE STRESS LARGE ENOUGH TO DISRUPT THE ACOUSTIC BOND BETWEEN SAID TRANSDUCER MEANS AND ONE OF SAID PIECES IN THE ABSENCE OF SAID STATIC COMPRESSSIVE STRESS, SAID VIBRATOR BEING DIMENSIONED TO VIBRATE 